"It was only a three tonne bridge so it shook him a bit," Waipa District Council contract engineer Eddie Shadrock said.

"The whole bridge could've gone. You have the river below - it's really scary."

The more than century-old Victoria St Bridge in Cambridge was closed about midnight Wednesday but was opened mid-morning to one lane for traffic coming from Leamington into the town centre.

Hamilton police Senior Sergeant Andrew O'Reilly said the man was following the GPS in his 42 tonne truck and trailer unit when he travelled onto the high-level bridge about 11.30pm. The 108-year-old bridge's maximum load weight per vehicle is three tonnes.

The full tanker heading from Tirau carrying insulated milk product went onto the curb scraping the bridge railings causing significant damage to main bridge in the Waikato town.

Waipa District Council project engineer Greg Wiechein , who was at the scene on Wednesday morning, said they would likely open the other lane when it was cleaned up, but the footpath would remain closed until it was fixed.

Wiechein said the handrail was severely damaged, parts of the rubber flashing around the base of the handrail has been ripped out and some of the fibre reinforced panels of the footpath need to be replaced.

Wiechein said the truck company's insurance would pay for the damage.

"It won't be cheap because we're going to have to get a bridge unit underneath to have a look at the damage to the footpath," he said.

"There's only one unit that can do it and they're based in Auckland."

The truck driver, from an Invercargill-based tanker company, was travelling from Tirau when he was believed to have diverted off State Highway 1, said Shadrock.

"He was going to come up Shakespeare and he got to the roundabout and couldn't negotiate it so went around the block to Cambridge," Shadrock said.

"But then he saw a sign to Te Awamutu. He followed that and once he was on the bridge he realised he shouldn't be on it, but couldn't back the truck out."

Shadrock was called about midnight and arrived to find the tanker had gone over the curb, blowing three tyres and damaging it's steering.

"He [the driver] was pretty remorseful - he had been talking to his boss as well. He was pretty worried about it."

The driver had been working for eight years and this incident shook him, Shadrock said.

He estimated the damage could cost $10,000-$12,000 to repair. The tanker company would be liable for costs.

"We won't know until the consultant gets here and checks the under-side of the bridge for any structural damage."